Forces In the Rough

R mg The block sits on the table. What forces are there ? The normal reaction The weight m is the mass of the block mg g is gravity (9.8 ms-2)

R mg If we start to push it what forces are there then? The normal reaction Friction A pushing force The weight m is the mass of the block mg g is gravity (9.8 ms-2)

R mg And if we start to pull it what forces are there then? The normal reaction Friction A pulling force The weight m is the mass of the block mg g is gravity (9.8 ms-2)

Worth Knowing F≤R Where  is a constant, which depends only on the roughness of the surface, and is known as the coefficient of friction.

Some worked examples 1. A particle of mass 10kg is placed on a rough horizontal plane. A horizontal force P acts on the particle. P is increased until the particle is on the point of sliding, which occurs when P=10N. Find the coefficient of friction between the particle and the plane . Click to reveal answer 1. Draw a diagram Resolve vertically R = 98 N Resolve horizontally F = 10 N Motion is just about to occur so F = R 10 = 98   = R F 10N 98N

And the next example is. . . . In the following situations a body of mass 10kg is placed on a rough horizontal plane. If  = ½ in each case determine whether motion will occur. Click to reveal answers R R 50 N R 60 N F 30 N F 70 N F 30º 98N 98N 98N Resolve vertically R = 50 + 98 = 148 N Fmax= 0.5 x 148 = 74 N F = 70 < 74 So no motion will occur Resolve vertically R + 60sin30º = 98 R = 68 N Fmax = 0.5 x 68 = 34 N F = 60cos30º = 52 > 34 N So sliding occurs Resolving vertically R = 98 N F = 30 < 49 (R) So no motion takes place

So off you go and do some yourselves Mrs. G’s book pg 64 exercise 4F Big Blue pg 182 q 1, 2, 3, 5 pg 183 enjoy